The present invention relates to a switching unit comprising:                a switch core fabric,        an input termination module coupled to said switch core fabric,        an output termination module coupled to said switch core fabric,said input termination module comprising:        at least one ingress queue adapted to backlog data units bound to said output termination module,said switching unit further comprising as forming part of said input termination module and of said switch core fabric, at least point-to-point transmission means adapted to convey data units from at least said at least one ingress queue to at least said output termination module, said at least point-to-point transmission means being considered, with respect to said at least one ingress queue and to said output termination module, as a virtual ingress-to-egress pipe connecting said at least one ingress queue to said output termination module.        
Such a switching unit is already known in the art, e.g. from the patent application entitled “Method for Sharing Internal Excess Bandwidth between Output and Input Termination Modules of a Switching Network”, No. 00402753, published by the European Patent Office (EPO) on Apr. 10, 2002.
To operate a switching unit efficiently, and to support controlled Quality of Service (QoS) differentiation in a bursty, unpredictable traffic pattern environment, the switching unit has to be complemented with a flow control mechanism. The granularity of the individual amounts of user traffic for which a flow control decision has to be made, the number of destinations and Classes of Service (CoS) to be distinguished, and the state of technology that can be used to implement the flow control, determine the rate and precision of the flow control decisions, and whether this flow control can be realized in a centralized or distributed manner. As the switching unit size and the applied flow granularity grow, either higher processing and communication speed, or more calculation and communication resources are required.
Credit based flow control grants every potential source of traffic in the switching unit a given limited amount of traffic it can allow into the fabric. The source can use the obtained credit at its own discretion. The credit is renewed on demand or on a regular basis. The calculation rule for granting the credit has to be fair over time to avoid giving an unjustified advantage to a particular physical path across the switching unit.
Internal Dynamic Rate-based Flow Control (IDRFC) is a credit based flow control, involving bandwidth negotiation between each and every input and output termination module within a fixed time schedule (IDRFC cycles).
The cited document discloses a switching unit that implements a weighted IDRFC algorithm for sharing an internal excess bandwidth among the input termination modules, according to bandwidth requests from the input termination modules. The bandwidth request and grant related to an input termination module linked by a virtual ingress-to-egress pipe to an output termination module are both calculated for a determined number of relative administrative weights corresponding each to a different CoS, with a different request and a corresponding grant for every weight.
Credit based flow control suffers from latency in the event of abrupt traffic increase. This latency is a consequence of the fixed measurement and calculation time cycles needed to ensure fairness and optimal use of the available bandwidth.
Referring back to IDRFC, 3 IDRFC cycles are necessary for accommodating a traffic pattern change:                nth cycle: measurement of the incoming traffic at every input termination module,        n+1th cycle: bandwidth calculation and negotiation between each and every input and output termination modules,        n+2th cycle: new bandwidth distribution scheme at every input termination module.        
If e.g. a step-shaped traffic pattern is applied at a particular input termination port, then incoming data units are backlogged for up to 2 IDRFC cycles before being scheduled towards the switch core fabric, and notwithstanding the fact that bandwidth may be immediately available.
Due to the quasi-linearly increasing calculation time required for increasing numbers of ports and CoS queues per port, scalability will have to be realized on one hand by calculation technology improvements, and on the other hand by increasing the cycle duration, which is the single remaining degree of freedom that allows to relax the calculation speed requirements.